This invention relates generally to methods and apparatus for modifying musical tones to achieve desired musical effects, and is more particularly concerned with modifying electrical tone signals in electrical musical instruments such as those of the organ type to produce celeste and other musical effects.
Celeste is the effect produced by playing two or more closely tuned tones together. In a pipe organ, celeste may be produced by sounding together sets of pipes which are naturally or purposely tuned slightly flat and slightly sharp, so that their sounds randomly move into and out of phase, with the true frequency of the note represented by the key played by the organist. Celeste therefore has slow "beating" associated with it, depending on the frequency spread between the tones involved; that is, the extent to which they are out of tune as compared to the reference note.
Various attempts have been made to utilize electrical phasing circuits to modify tone signals to produce chorus or celeste effects. A system described in Leslie, U.S. Pat. No. 3,372,225 utilizes rotary electrostatic devices for producing celeste or chorus effects. However, in this system the beat rates between the signals in two auxiliary electric-acoustic channels are constantly varying, and the same beat rate is applied to the whole musical instrument. The resultant sound is thus an undulating, varying sound, whereas in true celeste the beat frequency should remain constant throughout the period of time the note is played.
U.S. Pat. No. 3,489,843 describes apparatus for producing celeste animation in music wherein an electrical tone signal, or mixed signals, corresponding to the musical tone or tones desired to be modified, is passed through an artificial electrical transmission line, such as an audio frequency delay line, and a pair of scanning members in effect scan the delay line in a cyclic to and fro manner in such a way that as one scanning member is scanning the line in one direction, the other scanning mamber is scanning the line at the same speed in the opposite direction. The scanning of the artificial transmission line in one direction by a scanning member results in the production of a signal on the scanning member having a frequency f.sub.1 higher than that of the input signal, and scanning the line in the opposite direction by a second scanning member results in the production of a signal on the second scanning member having a frequency f.sub.2 lower than that of the input signal. The scanning members reverse their scanning direction at the ends of the artificial transmission line substantially simultaneously, so that when the signal produced on one scanning member changes from frequency f.sub.1 to frequency f.sub.2, the signal produced on the other scanning member changes from frequency f.sub.2 to frequency f.sub.1, such changes in frequency being effected abruptly. The result is the production of two substantially continuous new frequencies, f.sub.1 and f.sub.2, from one given input signal frequency.
The signals of alternately higher and lower frequency which are picked up by the two scanning members are preferably reproduced through separate loud speakers for converting the electrical signals into sounds.
Although this system is theoretically capable of producing acceptable celeste if the reversals of direction of scanning do not occur too frequently, there is a perceptible discontinuity in the output signals upon reversal, and its commercial implementation, which had a relatively short delay, did not produce particularly good celeste. Moreover, the apparatus is electro-mechanical in character, the elements of the artificial transmission line are bulky and relatively costly, making it somewhat undesirable for use in modern electric organs which are substantially all-electronic and implemented more and more with compact and increasingly less expensive integrated circuits. Although electronically variable delay lines are known and utilized in tone-modifying systems, since the operation of the system described in U.S. Pat. No. 3,489,843 depends for its operation on the simultaneous scanning of a fixed delay line in opposite directions, an electronically variable delay line cannot be directly substituted for the described combination of a fixed audio delay line and a system of stationary capacitor plates and a pair of movable scanning members, in the form of capacitor plates, mounted at the ends of a rotatable arm.
Known electronically variable delay lines are utilized in the tone-modifying system described in Doughty U.S. Pat. No. 3,749,837, wherein the delay line may take the form of a "bucket-brigade" analog shift register through which electrical representations of the magnitude of an input tone signal are periodically sampled, stored and shifted progressively from the input to the output of the shift register, thereby to delay the signal. The trigger pulses which time the sampling and shifting functions are frequency modulated by a low frequency-modulating signal to vary the time delay imposed, thus effecting a frequency modulation of the delayed input tone at the output of the shift register. If a msuical tone of given frequency is applied to the input of the Doughty system, and the frequency appearing at the output of the device measured as the clock frequency is changed, a certain amount of detuning, quite suitable for vibrato and tremolo purposes, results. The pitch of the output signal goes up and down as the clock frequency is modulated, depending on whether the clock frequency is above or below an average frequency. Thus, the Doughty device provides a relatively simple way of producing frequency-proportional vibrato. However, when it is attempted to so modulate the clock that the output frequency is detuned a constant percentage from the input frequency, the condition necessary to produce the celeste effect, it has been found that this could be accomplished only with an extremely complex modulating waveform. As a practical matter, oscillators conventionally employed as clock generators are usually of the voltage-controlled type, and when such an oscillator is used to modulate the clock frequency of a "bucket-brigade" shift register, it has been found that a given change in amplitude of the control voltage produces a much larger change in output frequency at the lower clock frequencies than at higher clock frequencies; that is, at the higher clock frequencies, less detuning for a given change in control voltage is achieved than at the lower clock frequencies. Thus, in order to produce a celeste effect, the waveform of the clock modulating signal would have to be such as to modulate the clock at a rate which increases rapidly as the clock frequency increases; to achieve this result, the modulating waveform would have to be of complex exponential shape, one very difficult to generate and to reproduce in practice.
An object of the present invention is to provide an improved, all-electronic system for modifying an electrical tone signal to produce celeste and other musical effects.